1. Field of the Invention
This application relates to electronic article surveillance systems, and more particularly to automated tuning of an electronic article surveillance antenna pedestal.
2. Description of the Related Art
Electronic article surveillance (EAS) systems are used to reduce theft of articles from a protected area, such as a retail store. The EAS system transmits an electromagnetic signal to establish an interrogation zone, which is typically located at the store's exits. An EAS tag adapted to respond to the transmitted signal when in the interrogation zone is attached to each article to be protected. The EAS system's receiver detects the EAS tag response. EAS tags attached to articles that have been purchased or are authorized for removal are removed or deactivated prior to the article passing through the interrogation zone. Therefore, the detection of an EAS tag within the interrogation zone indicates that an article is being removed without authorization, and appropriate action can be taken, such as setting off an alarm to warn personnel.
In EAS system installation, the installed antenna must resonate at the desired transmit frequency to transmit the maximum amount of energy into the interrogation zone. The antenna is connected to a pedestal, which contains matching capacitors and electronics, and both are connected to a power pack containing system electronics. The antenna is tuned according to the formula f0=½π(LC)1/2, where the capacitance is adjusted to tune the antenna to the resonant frequency. For example, pulsed magnetomechanical EAS systems, such as the FLOOR*MAX and PRO*MAX systems sold by Sensormatic Electronics Corporation, are tuned to about 58 kHz. Tuning is accomplished manually by a trained technician using an oscilloscope and other test equipment. The following is a typical tuning procedure:                1.—Connect an oscilloscope with a current probe to the transmitter antenna coil.        2.—Turn on the power pack and measure the current for both the top and bottom coils.        3.—Note the current amplitude readings.        4.—Turn off the power pack        5.—Guess if more or less capacitance in needed.        6.—Look up a jumper setting in the table and place the jumper settings accordingly.        7.—Set the jumper as specified onto the PC board        8.—Turn on the power pack and measure the current        9.—Note if the current increased or decreased.        10.—If the current increased the guess at step 5 was correct. If the current decreased the guess was incorrect and the jumper settings where wrong.        11.—Note the current amplitude readings for the top and bottom coils.        12.—Is the current maximized? If not got to step 4. If the current is maximized go to the next antenna.The above process is very time consuming and prone to errors as well as requiring expensive tools and specialized training. A more efficient, less expensive technique for antenna tuning is desired.        